The present invention relates generally to semiconductor integrated circuit devices and more particularly to an integrated programmable attenuator.
In many circuit applications it is necessary to vary the gain of an amplifier. For example, in the telecommunications field each transmit and receive circuit is coupled to an amplifier whose gain is adjusted to compensate for the attenuation characteristics of the transmission medium. Further, the amplifiers must be readjusted if the characteristics change significantly.
The simplest means of adjusting the gain of an amplifier 10, referring to FIG. 1, is to connect a continuously variable resistance 12 in the amplifier feedback circuit. The gain of the amplifier is approximately equal to the negative ratio of the feedback resistance to the input resistance, i.e., -(R.sub.F R.sub.I). The continuously variable resistance is typically a mechanical potentiometer, which is subject to mechanical wear and is a source of noise in a telecommunications system. Further, the amplifier gain must be manually reset each time the need arises.
In another circuit shown in FIG. 2, a series of resistors R.sub.A -R.sub.D are connected to the input and feedback resistors R.sub.I and R.sub.F and coupled through a switch 14 to an amplifier 16. The gain of amplifier 16 may thus be varied in discrete steps. For example, the gain of amplifier 16 with switch 14 in the position shown is approximately -(R.sub.F +R.sub.B +R.sub.C +R.sub.D)/(R.sub.I +R.sub.A). However, the implementation of this circuit with discrete components has many of the shortcomings of the previously described circuit.